Imager devices are used in digital cameras, wireless devices with picture taking capabilities, and many other applications. Cellular telephones and Personal Digital Assistants (PDAs), for example, are incorporating imager devices for capturing and sending pictures. Imager devices include image sensors that use Charged Coupled Device (CCD) systems, Complementary Metal-Oxide Semiconductor (CMOS) systems, or other solid-state systems.
As shown in FIG. 1A, imager devices may be formed on a wafer 1 that may include one or more PN junctions across the wafer 1. In the FIG. 1A example, wafer 1 includes a silicon substrate 12 with an n-type layer 6 over a p-type layer 3. A p-type layer 9 is over the n-type layer 6 and an n-type layer 7 is over the p-type layer 9. The wafer 1 includes a plurality of die 10 fabricated therein. Each die 10 includes various devices 14. As an example, die 10 may include imaging devices 14.
The wafer 1 is cut at the saw or scribe line 2 to singulate each die 10. When the wafer is cut, damage can occur at the cut edges 13 (FIG. 1B) of the substrate 12. In particular, damage at the PN junction 16 (FIG. 1B) along the cut edge 13 can lead to charge leakage paths across the junction 16.
FIG. 2A is a schematic diagram of the electrical equivalent of a portion of the wafer 1 prior to cutting. The first n-type layer 6 may be tied to a voltage source, VAA and each die 10 includes a diode at its PN junction 16. FIG. 2B is a schematic diagram of the electrical equivalent of a portion of each die 10 after cutting. As shown in FIG. 2B, damage along the cut edge 13 creates leakage pathways 17 that did not previously exist. In some instances, charge leakage through pathways 17 may adversely affect performance of devices 14, e.g., imager devices. Similar problems may exist for other electronic devices and/or circuits fabricated on a semiconductor substrate 12 having a junction between a P and N region at a scribe line.
Embodiments described herein provide structures and methods for preventing charge leakage across P and N junctions at a cut edge of a die.